3 ms·

> the grid _drives the generator as a motor_ Can you expand on this/link a relevant article? I'm a layman and this sounds really interesting!

xenadu02 2y ago
Take a little hobby DC motor and hook it to a battery. It will spin. Now hook it to a small LED and turn the shaft. The LED will light up. In fact hook both up. The battery will spin the motor and light the LED based on its voltage/charge state. But if you start spinning the shaft you will begin taking over from the battery. Spin even faster and the battery will start charging. Stop trying to spin the shaft and watch the LED dim and the motor slow down as the battery takes over again. Fundamentally electromagnetism is a unified force. A changing electric field induces a magnetic field: the electricity creates a magnetic field inside the motor-generator causing the rotor's electromagnet or permanent magnet to spin to align with that field. For a DC motor halfway through the rotation the polarity switches causing the rotor to keep spinning to align with the new magnetic field (AC doesn't need this because it is varying on its own). The motor-generator is operating in "motor" mode here because the magnetic field changes first and the rotor is trying to catch up to it ultimately consuming energy. Now spin a motor's shaft putting energy into the system. The rotor has a magnetic field either because its an electromagnet or a permanent magnet. You are spinning the rotor shaft so by definition the magnetic field coming off the rotor also spins. Thus the stator sees a continuously varying magnetic field which - you guessed it - induces an electric field in the stator windings. In reality this is a continuously varying effect based on whether the rotor's motion is leading or lagging the magnetic field (really it is trying to lead or lag but never gets far otherwise it would burn up). The short version is a generator at "idle" but synchronized is a motor. The grid is providing the power to spin that motor (or the attached engine/steam turbine is providing just enough rotation of the shaft to keep the generator synchronized but no more). As the generator begins to "provide" power more torque is put into the shaft's rotation. This causes the magnetic field to want to lead the electric field, transferring energy into the electric field. The entire grid strongly resists the shaft actually spinning faster though. The mechanical energy that wants to make the rotor go faster is siphoned off by the magnetic flux and electric fields. Electric field steals energy from the magnetic flux. Reduced magnetic flux steals energy from the rotor's angular momentum to replenish itself (if it couldn't the rotor would slow down). The rotor gets angular momentum from the connected source of torque like a steam turbine. The Cycle of Life ... or something. If you suddenly cut the transmission lines and took no action all that torque would overspeed the generator very quickly though because no electric current could flow so the electric field just builds up (stealing energy from the rotor) then gives the energy right back as more magnetic flux (speeding up the rotor). This would let the rotor just go faster and faster while also heating everything up.
wglb 2y ago
If you have two AC generators connected to a load and one of them shifts is phase any amount ahead of the other one, the second one becomes a motor. Power flows in proportion to the difference in phase angle.

xenadu02 2y ago

Take a little hobby DC motor and hook it to a battery. It will spin. Now hook it to a small LED and turn the shaft. The LED will light up. In fact hook both up. The battery will spin the motor and light the LED based on its voltage/charge state. But if you start spinning the shaft you will begin taking over from the battery. Spin even faster and the battery will start charging. Stop trying to spin the shaft and watch the LED dim and the motor slow down as the battery takes over again. Fundamentally electromagnetism is a unified force. A changing electric field induces a magnetic field: the electricity creates a magnetic field inside the motor-generator causing the rotor's electromagnet or permanent magnet to spin to align with that field. For a DC motor halfway through the rotation the polarity switches causing the rotor to keep spinning to align with the new magnetic field (AC doesn't need this because it is varying on its own). The motor-generator is operating in "motor" mode here because the magnetic field changes first and the rotor is trying to catch up to it ultimately consuming energy. Now spin a motor's shaft putting energy into the system. The rotor has a magnetic field either because its an electromagnet or a permanent magnet. You are spinning the rotor shaft so by definition the magnetic field coming off the rotor also spins. Thus the stator sees a continuously varying magnetic field which - you guessed it - induces an electric field in the stator windings. In reality this is a continuously varying effect based on whether the rotor's motion is leading or lagging the magnetic field (really it is trying to lead or lag but never gets far otherwise it would burn up). The short version is a generator at "idle" but synchronized is a motor. The grid is providing the power to spin that motor (or the attached engine/steam turbine is providing just enough rotation of the shaft to keep the generator synchronized but no more). As the generator begins to "provide" power more torque is put into the shaft's rotation. This causes the magnetic field to want to lead the electric field, transferring energy into the electric field. The entire grid strongly resists the shaft actually spinning faster though. The mechanical energy that wants to make the rotor go faster is siphoned off by the magnetic flux and electric fields. Electric field steals energy from the magnetic flux. Reduced magnetic flux steals energy from the rotor's angular momentum to replenish itself (if it couldn't the rotor would slow down). The rotor gets angular momentum from the connected source of torque like a steam turbine. The Cycle of Life ... or something. If you suddenly cut the transmission lines and took no action all that torque would overspeed the generator very quickly though because no electric current could flow so the electric field just builds up (stealing energy from the rotor) then gives the energy right back as more magnetic flux (speeding up the rotor). This would let the rotor just go faster and faster while also heating everything up.

wglb 2y ago

If you have two AC generators connected to a load and one of them shifts is phase any amount ahead of the other one, the second one becomes a motor. Power flows in proportion to the difference in phase angle.